Stereotactic radiosurgery (SRS) is a specialized form of radiation therapy that focuses small radiation beams on precisely localized areas of the brain that are known or thought to contain diseased tissue. Current SRS systems, for example as illustrated in FIG. 1, mostly use photon beams. Photon radiation beams are delivered to the target area from many different angles with extreme accuracy. The method effectively treats abnormal areas of tissue with negligible damage to nearby healthy tissue. Photon radiation is most often used to treat pituitary adenomas, brain metastases, arteriovenous malformations, and the like. For example, the University of Iowa Hospitals and Clinics treats about one hundred fifty (150) patients each year, roughly around 10-15% of the total radiation therapy patients, using this SRS technology.
The most popular photon SRS systems are linear accelerator (Linac) based systems or GammaKnife systems. In GammaKnife radiosurgery, close to two hundred (200) tiny radiation beams from radioactive Co-60 sources are used. Comparatively, Linac radiosurgery delivers radiation from one source in multiple narrow, collimated photon beams known as arcs, with a tertiary collimator, known as a “cone.” During the procedure, the patient is localized to pre-determined positions by a mechanical system with 0.3 mm accuracy.
Patients undergoing SRS through current methods (e.g., Linac and GammaKnife systems) are first fitted with a head ring or head frame 10, as shown in FIG. 2. The head frame 10 attaches to the head of the patient via pins/screws at various points. The head frame 10 must be tightly secured to the skull of the patient in order to establish a rigid relationship between the head and head frame 10 for the duration of the treatment. The head frame 10, along with a localization box (not shown) used during computed tomography (CT) imaging is used to establish a precise, three-dimensional coordinate system of the patient's brain. Later, when the patient is secured to the treatment table or floor stand through this head frame 10, the brain lesion can be precisely positioned at the desired location relative to the Linac beam.
While photon beams are proven for treating small, relatively uniformly shaped brain lesions, the risk of side effects increases when photon beams are used to treat large, irregularly shaped targets. Therefore, the photon SRS techniques described above becomes impractical for large target volumes.
Proton radiation, due to proton beams superior depth dose properties, has an inherent advantage in treating large, irregularly shaped lesions in comparison to photon radiation. A proton naturally releases the vast majority of its energy in matter near the end of its path, illustrated in FIG. 3, called the Bragg Peak. By manipulating proton energies, the Bragg Peak can be placed within the target volume, maximizing tumor dose but minimizing healthy tissue dose, in contrast to photons (shown along the x-ray deposition curve) that have substantial entrance and exit doses, as illustrated in FIG. 4. An advantage of proton therapy is the minimal entrance dose and virtually absent exit dose when the Bragg peak is placed at tumor.
However, placing the Bragg peaks within the tumor of various depths requires the ability to adjust proton energy. Current medical proton systems have a lower energy threshold of approximately 70 MeV, which is too high for direct treatment of shallow lesions located less than four centimeters beneath the skin, which make up a large percentage of brain lesions. In fact, the average adult human head is about sixteen centimeters wide, and the shallowest four centimeters consists of more than 50% of the brain volume where lesions may occur. For example, between May 2009 and September 2009, 25% of brain SRS patients treated at University of Iowa Hospitals and Clinics had a portion of their lesions within 3 mm of the inner skull. In addition, the shallowest four (4) centimeters of the brain is the site of many capillaries where metastasized tumors are often found due to the capillaries decreased diameter, increasing the difficulty of the tumors to flow through capillaries.
In order to treat shallow tumors and manipulate the depth at which the Bragg peak is placed, a range-shifting device is needed to lower the proton beam energy when entering the patient. It is estimated that one-third of all proton therapy procedures will require a range-shifting device, often called a range shifter. Therefore, a range shifter is necessary to treat these lesions in proton SRS.
However, existing range shifters are slabs of tissue-mimicking plastic that are often placed in the beam-line/beam exit window well above the head of a patient. Such placement leaves a considerable distance, as large as forty (40) cm, between the range shifter and the patient skin. Such distances increase the amount of lateral growth of the proton beam due to multiple Coulomb scattering inside of the range shifter. Multiple coulomb scattering refers to the gradual spread of the angular distribution of protons that arises from thousands of small electrostatic deflections by atomic nuclei. A large air gap between range shifter and patient increases the unwanted radiation dose to patient, and more specifically healthy tissue surrounding the targeted lesion, and decreases the dosimetric advantage of proton SRS. For example, FIGS. 5A-B show a simulation study that compared the radiation dose distribution applied to a tumor (outlined in yellow) for a range shifter five (5) cm away from patient head (FIG. 5A) and one range shifter forty (40) cm away from patient head (FIG. 5B). FIG. 5B shows that when a range shifter is far away from the head of a patient, a good portion of the radiation dose is delivered to healthy brain tissues outside the tumor, whereas FIG. 5A shows the radiation dose more focused around the tumor. Further, as shown in FIG. 6, there are times when proton beams are actually worse than photon beams for brain SRS application. For example, when proton beams have a lateral size of 7.1 mm or larger, proton beams may be more damaging than photon beams. In fact, proton SRS may only be superior to photon SRS when lateral size of proton beam is smaller than 4.3 mm.
Therefore, there is a need for a range shifter device that allows for the range shifting necessary in proton SRS for shallow lesions while minimizing the lateral growth of the proton beam due to scatter.